In seismic prospecting using simultaneous vibratory techniques, a series of seismic energy sources (e.g., vibrators) are employed to transmit seismic signals into the earth. Part of these signals are reflected from interfaces between subterranean strata, and/or refracted within strata, back to the surface of the earth, where they are detected by one or more receivers. The time taken for a signal to pass from a particular vibrator to a particular receiver gives an indication of the length of travel of the signal between that vibrator and that receiver, from which the structure of geological formations may be deduced.
With simultaneous vibratory techniques vibrators collectively and simultaneously impart energy into the earth from multiple source locations. Thus, each receiver detects refracted and reflected energy which has been emitted by the whole series of vibrators. (As an aside, “simultaneous” sweeping entails overlap between the sweeps. The sweeps may or may not begin and/or end at the same time so long as there is some portion of overlap between the sweeps.) The data recorded at each receiver must then be processed so that the signal due to each individual vibrator can be separated out.
This separation may be achieved by each vibrator performing multiple “sweeps” or “shots,” where the relative phase of the signals emitted by the vibrators are varied between vibrators and between shots (although the relative phase is held constant through the duration of the respective sweeps). This can be illustrated by the case of two vibrators, twice operated simultaneously. If they are operated in phase with each other for the first sweep, but 180° out of phase for the duration of the second sweep, the receiver will record two signals (i.e., the “relative phase angle” between the two vibrator drive signals is 180° out of phase for the duration of the second sweep). (As an aside, all four of the sweeps may be driven, via a “driving signal” or “control signal,” at the same amplitude with a swept-frequency, possibly with tapered ends because the amplitude has to be ramped up at the start and ramped down at the end. The control signal may begin at low frequency and finish at high frequency, and the frequency may increase linearly with respect to time.) The resultant recorded signals may be added together to determine the signal arriving from the first vibrator, or subtracted to determine the signal arriving from the second vibrator.
While signal separation has improved over the years, noise minimization and signal separation can still be improved to better locate subterranean areas of interest.